Dynamic particle capture and adhesion have long been recognized to be important in technological applications ranging from aerosol handling to tribology. This significance now extends into the biological arena. For instance, flowing cells and bacteria are captured on surfaces when neutrophils roll on injury-activated endothelium, during cancer metastasis, and upon initiation of viral and bacterial infection. These behaviors have been reproduced ex-vivo with both cells and rigid round particles functionalized with adhesion proteins to demonstrate underlying principles. Sophisticated biomaterials that achieve similar dynamic and selective control of cells are, therefore, logical candidates for the next generation of diagnostic devices and systems for the refinement of cell suspensions for tissue engineering.
Currently, most research on cell-biomaterial adhesion focuses on the development of materials that bind cells via relatively dense receptor placement on a surface (or tethered from the surface), or relatively concentrated distributions of receptor-bearing nanoparticles. In quiescent conditions, there is an optimal receptor loading for cell proliferation or differentiation: Multiple contacts between the cell and receptors on biomaterials stimulate the formation of an oriented actin network within each cell. Likewise, multiple interaction sites between cells comprise the mechanism for infection, immune response, fertilization, and differentiation of cells. Indeed, Nature seems to have good reason to rely on many weak interactions rather than a few strong ones for cell adhesion and signaling. At the opposite extreme, however, there is technological value to biomaterial-cell interactions generated by one or a few strong contacts. With reports of a receptor threshold for communication between cells, materials that capture cells with a limited number of contacts may be able to do so without the cell's knowledge. That is, limiting the numbers of contacts may facilitate cell manipulation without signaling. Likewise, surfaces which trap cells and particles with just a few or even a single contact can also provide highly sensitive detecting and sensing elements.